<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      var a = '111'
      console.log(typeof a) // string

      var a = 111
      console.log(typeof a) // number

      var a = false
      console.log(typeof a) // boolean

      var a
      console.log(typeof a) // underfined

      var a = null
      console.log(a) // Object

      //结果一定是字符串  string类型
      console(typeof typeof a) //typeof typeof a
      console.log(typeof a + 100) // number100
      console.log(typeof (a + 100)) //number
    </script>
  </body>
</html>
